Desalination of saline water resources has been receiving much attention because of the increasing demand for fresh water in water-starved regions around the world. Available desalination technologies can be broadly categorized in three different groups.
A first approach is to use thermal means to change the phase of the water (from liquid to vapor, or from liquid to solid), and then separate the pure phase from the remaining salt solution. The separation step can be followed by recovery of thermal energy for reuse, as the vapor or solid phase changes to the liquid phase.
In a second approach, salt and water are physically separated by means of a semi-permeable membrane, under the influence of a chemical potential gradient which may be effected by application of pressure, by a concentration gradient or by an electrical potential.
A third method is chemical means to produce desalinated water which include ion exchange, liquid-liquid extraction, separation by formation of gas hydrates, or other precipitation mechanisms.
It is believed that all currently known desalination methods use one or more of the above three basic approaches. A number of these processes for desalination have been commercialized. These include thermal distillation, reverse osmosis, and electrodialysis. However, all of these desalination processes suffer from a major handicap, namely, costly energy consumption.
Thermal desalination processes require energy ranging from 20 kJ/kg to more than 200 kJ/kg. Energy requirements for reverse osmosis depend on concentration, and range from about 10 kJ/kg for brackish water to about 20 kJ/kg for seawater. According to Andrews, W. T., W. F. Pergande and G. McTaggarat (2001), “Energy Performance Enhancement of 950 m3/d Seawater Reverse Osmosis Unit in Grand Cayman”, Desalination 135: 195-204, even with the latest state-of-the-art technology, the energy requirement for reverse osmosis is still about 8.4 kWh/1000 gallon (2.2 kWh/m3).
While other costs are important, energy cost is the decisive factor preventing wide use of desalination processes. According to a recent estimate, for a reverse osmosis desalination plant, the cost of electric power constitutes 44% of the total cost of producing desalinated water, while the fixed charges or capital costs are 37% of the total cost. This estimate was reported in USBR and SNL (United States Bureau of Reclamation and Sandia National Laboratories). (2003) “Desalination and Water Purification Technology Roadmap: A Report of the Executive Committee. Desalination & Water Purification Research & Development Report #95”, Denver, Colo.: United States Department of the Interior, Bureau of Reclamation, Water Treatment and Engineering Group.
The USBR and SNL report also indicated that the minimum energy that is thermodynamically required for desalination of seawater containing 3.5% solution of sodium chloride due to osmotic pressure is 0.82 kWh/m3.
There have been efforts to produce new types of improved membranes and equipment so that energy requirements are reduced, but these efforts alone cannot dramatically reduce the energy consumption figure. There is therefore a need for a new technology which addresses the energy problem in such a way as to effect a paradigm shift in energy consumption.